U.S. Pat. No. 4,923,085 sets forth in detail the general state of the art relating to container closures, and specifically defines a closure system utilizing a rocking lid with a depending flange which directly seals to the open mouth of a container. The lids to which the patent is directed are generally intended for use on relatively large-mouthed containers of the type normally found in the kitchen for the storing of foodstuffs. As such, and as the opening of the lid is effected by a physical rocking or pivoting of the lid about fulcrum points, the lid itself must be of a substantially rigid construction, formed for example of a polycarbonate or polystyrene. The container will preferably be formed of a similar rigid material for appropriate support of the contents.
The sealing normally achieved between the rigid container and the rigid locking lid has been found to be effective for general purposes if close manufacturing tolerances are maintained. However, a more effective sealing is frequently desired or in fact found to be necessary for a variety of products and related storing conditions.
This necessity for a more effective sealing of the lid to the container, while maintaining the highly desirable rocking nature of the lid, gives rise to a substantial problem. Basically, if a softer plastic material such as low-density polyethylene or polypropylene is used to form the lid, it is possible a more intimate seal will be achieved. However, the lid will no longer possess the high degree of rigidity need to effect the essential rocking movement thereof to allow for an opening of the lid through pressure at a designated point thereon. Rather, the flexible lid, as with known lids of a highly flexible nature, would have to be peripherally peeled from the container by the fingers grasping the edge of the lid peripherally thereabout. As such, the distinct advantages derived from the provision of a rocking lid would not be achieved.